exhaust backfire

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Jbarker

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Hi fellas-
I hope someone can help me here, as I've been fighting this problem for a year!
I have a 71 Dart with 383 ->489 Source stroker kit, 10:1 compression, Hughes solid lifter cam, 2" tti headers, holley HP 850 carb, stealth heads, Mallory ignition box and coil, etc. I have a really loud exhaust backfire that occurs ONLY out the passenger exhaust, ONLY at idle, ONLY when the engine is warmed up. It occurs every 5-15 seconds. It's terrible though. It scares people's pets, cops are giving me dirty looks, babies are crying, gearheads laughing at me...not good. The car runs and drives great. Starts easy, excellent power. Sometimes idle is a little rough when hot, but not bad. No driveability problems.
This happened to me once before years ago. I traced it to a bad plug, replaced it, and all was well. Now it's happening again, so I replaced all the plugs, and no better. I replaced all the wires, no better. I have had new plugs that were bad out of the box before, so I tried replacing the 4 plugs on the passenger side again, and no luck. Still backfires. I checked the timing, and it was 10 degrees advanced. I set it to 12 deg BTDC, and the problem persisted. I checked the transfer slot on the carb to make sure it wan't over-exposed and dripping fuel, but it was right on. 4 corner idle was adjusted with no improvement. I took it as lean as I could go, then later tried a little richer. No change with any of it. I then swapped the carb with an edelbrock 800 Thunder AVS I had. The problem was marginally better, but barely. I still had a loud backfire out pass side exhaust at a hot idle. I thought maybe I had a vacuum leak, checked for one, found none, and changed the valley pan gasket anyway because by this time I am grasping at straws. Problem remained. I checked exhaust system for leaks, and it looked good aside from a ticking at the pass side header flange. I replaced the header gasket at the head with a tti copper gasket installed dry. Header leak was much better, but not gone. Backfiring problem no better at all. Now I moved on to compression test looking for a sticky/burnt exhaust valve. I did the test last fall, and don't remember the numbers, but compression was good, all cylinders within a few percent of eachother. Last week I did a leakdown test (still pursuing the possibility of a bad valve). All cylinders were perfectly equal at 6% leakdown. So what next?! I have a few ideas, and I may mention them later, but for now I would love anyone's suggestions on this. Thanks in advance for any help!
Desperate in Buffalo,
-Jay
 
Find some one with a scope and eliminate the ignition. I would say a bad exhaust valve but the leakdown is good.
 
Repair the exhaust leak.
It's likely sucking air, and igniting unburnt fuel in the pipe.
Does it pop on decel to a stop or downhill ?
 
THANK YOU for listing the year and engine and all the things you have tried, so refreshing.

Just some thoughts...

Since it only happens at idle, when hot, points me in two directions,
  1. An exhost valve that sticks open occasionally but only when hot. Could be a clearance issue, as in when something, (lifter, rocker, valve) heats up it grows and the friction is such that it sticks open just a tad. Could also be a lifter pumping up, heat related
  2. The electronic ignition is malfunctioning when it gets heated up throwing spark at the wrong time.
You mentioned it runs rough at idle at times. Does that happen at the same time as the backfires.
 
Jbarker said:
Hi fellas-
I hope someone can help me here, as I've been fighting this problem for a year!
I have a 71 Dart with 383 ->489 Source stroker kit, 10:1 compression, Hughes solid lifter cam, 2" tti headers, holley HP 850 carb, stealth heads, Mallory ignition box and coil, etc. I have a really loud exhaust backfire that occurs ONLY out the passenger exhaust, ONLY at idle, ONLY when the engine is warmed up. It occurs every 5-15 seconds. It's terrible though. It scares people's pets, cops are giving me dirty looks, babies are crying, gearheads laughing at me...not good. The car runs and drives great. Starts easy, excellent power. Sometimes idle is a little rough when hot, but not bad. No driveability problems.
This happened to me once before years ago. I traced it to a bad plug, replaced it, and all was well. Now it's happening again, so I replaced all the plugs, and no better. I replaced all the wires, no better. I have had new plugs that were bad out of the box before, so I tried replacing the 4 plugs on the passenger side again, and no luck. Still backfires. I checked the timing, and it was 10 degrees advanced. I set it to 12 deg BTDC, and the problem persisted. I checked the transfer slot on the carb to make sure it wan't over-exposed and dripping fuel, but it was right on. 4 corner idle was adjusted with no improvement. I took it as lean as I could go, then later tried a little richer. No change with any of it. I then swapped the carb with an edelbrock 800 Thunder AVS I had. The problem was marginally better, but barely. I still had a loud backfire out pass side exhaust at a hot idle. I thought maybe I had a vacuum leak, checked for one, found none, and changed the valley pan gasket anyway because by this time I am grasping at straws. Problem remained. I checked exhaust system for leaks, and it looked good aside from a ticking at the pass side header flange. I replaced the header gasket at the head with a tti copper gasket installed dry. Header leak was much better, but not gone. Backfiring problem no better at all. Now I moved on to compression test looking for a sticky/burnt exhaust valve. I did the test last fall, and don't remember the numbers, but compression was good, all cylinders within a few percent of eachother. Last week I did a leakdown test (still pursuing the possibility of a bad valve). All cylinders were perfectly equal at 6% leakdown. So what next?! I have a few ideas, and I may mention them later, but for now I would love anyone's suggestions on this. Thanks in advance for any help!
Desperate in Buffalo,
-Jay
Click to expand...
I believe you found the problem in the exhaust leak as stated by inertia.
 
Thanks everyone- Only a few hours after my post and now I already have some things to look into!

Halifax- I like your idea of checking ignition with an oscilloscope. I will look into that.

Inertia- It never pops on decel or going down hills. It always backfires at idle. Also, when I first start to accelerate, it often backfires. Exactly the moment the throttle blades are eased open.
I suppose I could coat the copper header gasket with hi temp RTV, but I hate to make a mess of it if that isn't the problem. There is not much room to work in there. I have had header leaks on many cars (actually every car with headers I ever owned LOL), and have never had it cause a backfire. Even this car specifically has had worse header leaks in the past over the years, and it never backfired. So that makes me skeptical that it's the problem, though of course it may be. I will keep that possibility in mind.

Dana67dart- I agree with you that it could be a valve that only sticks when it gets hot. If that is the case, perhaps the leakdown test was normal only because the engine was cold?? Maybe the valve simply wasn't sticking because the engine was cold?
So what next? I think it would be near impossible to do that leakdown test on a hot engine (getting around those 2" primaries when they're hot, burning my hands, melting the leakdown tester hose, etc...)
Even pulling a head- maybe everything would seem fine because the head was cold...?
I also wondered if the electronic ignition box was the culprit, but it seems less likely since it only backfires out the passenger side. Anyways-you asked if it idles rough at the same time it backfires. Yes. It backfires when it's warmed up and the idle does seem to get worse as the engine gets hotter.

Thanks again for the advice guys. Much appreciated. Any other advice is welcome! I will keep you posted as this progresses.
-Jay
 
Just a guess, one exhaust cam lobe went flat. Sucks in fuel and air for good compression, but can't get out of the cylinder when exhaust tries to open.

So when the intake valve on that cylinder opens again, the pressurized charge heads up the intake and finds its way out one of the other open valves or up out the carb, taking a fuel air mixture with it.

Good luck . . .
 
^^^^^^ This was also my initial thought ^^^^^^^^^

Any nice shiny metallic metal during an oil change. I would pull pass side valve cover and set up a dial indicator to check lift at each rocker before ripping a head off.
 
OK this is a "Brain Tease" trying to figure this one out.

A 2 beer problem . . .

So to start with you probably have a performance solid lifter cam with adjustable roller rockers. A performance cam that comes on at 2500 rpm and peaks at 7500.

Giving you have a lope at idle, which means that intake and exhaust valves are open at idle waiting for you to rev it up to 2500 rpm so the flow through the cylinder can work efficiently then.

Starting my second beer now . . .

So if this is the case the open valves at idle present the opportunity to introduce unburnt fuel air mixture into the exhaust. This is half of the Bang Backfire equation.

OK next it is only happening on the passenger side, same side as the leaky header at the head mount flange.

As the pulse of the expanding gasses make it past the header flange down the tube to the collector 2,4,6,8; 2,4,6,8 . . . This creates a suction (or vacuum) at the headpipe flange area where the leak is allowing atmospheric air to get sucked in to meet up with the hot expelling exhaust gasses.

Just the right conditions and it produces a Bang, burns all the unburnt gases out of the downwind exhaust. Then 5 -15 seconds later the fuel air mixture in the exhaust builds up again and Bang ! Backfire again, repeat, repeat, until the rpms pick up and the cylinders and the cam start burning more efficiently.

Like one of the guys was saying the header flange leak is allowing air into the exhaust creating a flashpoint for the exhaust gasses to go Bang, instead of sending them down the exhaust pipe out the back like the driver's side is doing.

Willing to bet you do not have an H-Pipe or X-Pipe exhaust, this straight through dual exhaust allows each side of the exhaust to work independent of the other.

Hope this helps.

Dang I need a beer . . .
 
Last edited:
That does help. Thanks. For the record, I do have an H pipe exhaust. Also, I checked the valve lash on the passenger side today and thankfully it is spot on. So I am glad to know that I don’t have a wiped out lobe on the cam.
 
If you do have solid lifters, perhaps your valve lash is just a tad too tight, cold it is ok, hot it keeps a valve open just a tad.

My 67 273 has one valve that when cold ticks, once it is warmed up it stops ticking, just a tad too much lash.
 
George Jets said:
Just a guess, one exhaust cam lobe went flat. Sucks in fuel and air for good compression, but can't get out of the cylinder when exhaust tries to open.

So when the intake valve on that cylinder opens again, the pressurized charge heads up the intake and finds its way out one of the other open valves or up out the carb, taking a fuel air mixture with it.

Good luck . . .
Click to expand...

Thot he said it was out the exhaust , not the intake ----------
 
Jbarker said:
That does help. Thanks. For the record, I do have an H pipe exhaust. Also, I checked the valve lash on the passenger side today and thankfully it is spot on. So I am glad to know that I don’t have a wiped out lobe on the cam.
Click to expand...

My thots are , ''fix the exhaust leak'' , and reduce the back pressure in ur exhaust system , less back pressure will help w/ blowing gaskets .
I have reused header gaskets on 3 diff. engines , (383 sbc , 406 sbc , and now a 505'' wedge ) , the headers have no welded ring on the head side of the flange, and haven`t had a leak yet , I dont have much to very little back pressure .
 
Seems like a sticking valve.
I would set valve lash a little loose to see if its one thats not closing when it gets hot. But if it was a valve issue, the vacuum gauge would show it.

if it only takes 15 seconds, i’d pull one plug wire at a time, see if its connected to one particular cylinder. But all 8.
 
It's not an exhaust lobe going flat. Something causing an exhaust valve not to open would make the engine pop through the carburetor, not the exhaust. Whatever it is, the exhaust valve has to be open for a pop to get into the exhaust system. Popping in the exhaust is a lean condition "somehow". Whether it's a cracked spark plug, bad wire, exhaust leak or "whatever" something is causing that side to lean out and pop into the exhaust.
 
I has been my experience that valves just dont stick at idle and when they do they break a rocker arm and/or bend push rod which is very easy to find.
When you replace the header gaskets use Percy's dead soft aluminum gaskets they are great and I have never had one leak. If possible chase the header bolt threads and clean the threads good and use new header bolts.
IMO I think you may have a combination of issues.
Are you runnig an MSD ignition?
 
Mallory hyfire box and coil. Distributor cap has already been replaced. Exhaust is dual 3”.
 
Just for part of the equation, what are the camshaft lift and duration numbers ?

Make and part number of camshaft ?

Want to see how big the overlap is at idle when both intake and exhaust valves are open at the same time.
 
I left out a burned exhaust valve. That's a possibility, but that cylinder would be skipping. If it's just an intermittent pop and it's not skipping, it's not a burned exhaust valve.
 
Yet another possibility.....lol.....is a weak or broken valve sprAng.
 
How about a leaky header collector gasket on the passenger side. Allowing fresh air to get sucked into the passenger side exhaust pipe.

If it is running lean like RRR said, the addition of more air could make it pop on the way out to the back.

________

Also is the muffler on the passenger side buldging out from the backfire pressure?

Bad backfires in the exhaust system can split the mufflers wide open at the seams.
 
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